PerfectNumber.java

// A perfect number is a positive integer which is equal to the sum of its proper positive divisors. Proper positive divisors are positive integers that fully divide the perfect number without leaving a remainder and exclude the perfect number itself.

// For example, take the number 6: Its proper divisors are 1, 2, and 3 (since 6 is the value of the perfect number, it is excluded), and the sum of its proper divisors is 1 + 2 + 3 = 6.  

// Therefore, 6 is a perfect number (as well as the first perfect number).  

// Write a method named isPerfectNumber with one parameter of type int named number.  

// If number is < 1, the method should return false.  

// The method must calculate if the number is perfect. If the number is perfect, the method should return true; otherwise, it should return false.  

// EXAMPLE INPUT/OUTPUT:  

//  isPerfectNumber(6); should return true since its proper divisors are 1, 2, 3 and the sum is 1 + 2 + 3 = 6
// isPerfectNumber(28); should return true since its proper divisors are 1, 2, 4, 7, 14 and the sum is 1 + 2 + 4 + 7 + 14 = 28
// isPerfectNumber(5); should return false since its only proper divisor is 1 and the sum is 1 not 5
// isPerfectNumber(-1); should return false since the number is < 1

public class PerfectNumber {
    public static boolean isPerfectNumber(int number) {
        if (number < 1) {
            return false;
        }

        int sum = 0;
        for (int i = 1; i < number; i++) {
            if (number % i == 0) {
                sum += i;
            }
        }

        return sum == number;
    }

    public static void main(String[] args) {
        System.out.println(isPerfectNumber(6));  // true
        System.out.println(isPerfectNumber(28)); // true
        System.out.println(isPerfectNumber(5));  // false
        System.out.println(isPerfectNumber(-1)); // false
        System.out.println(isPerfectNumber(15)); // false
    }
}